Flat cathode ray tube with increased deflection sensitivity



April 6, 1965 M. R. NAMORDI ETAL 3,177,395

FLAT- CATHODE RAY TUBE WITH INCREASED DEFLECTION SENSITIVITY Filed April4, 1963 6 Sheets-Sheet 1 FIGJ A SOURCE OF v HDRIZONTAL DEFLECTiONINVENTORS' SHGNAL MOOSHI R. NAMORDI,

SVEND E. HAVN,

THEIR ATTORN EY.

April 6, 1965 M. R. NAMORDI ETAL 3,177,395

FLAT CATHODE RAY -TUBE WITH INCREASED DEFLECTION SENSITIVITY Flled Aprll4, 1963 6 h h et 2 SOURCE OF HORIZONTAL DEFLECTION SIGNAL INVENTORSMOOSHI R. NAMORDI, SVEND E HAVN,

BY W

THEiR ATTORNEY.

A ril 6 1 6 M. R. NAMORDI ETAL q p 9 5 FLAT CATHODE RAY TUBE WITHINCREASED DEFLECTION SENSITIVITY 'Filed April 4, 1963 6. Sheets-Sheet I5m s.u v L8 xv SOURCE OF VERTICAL DEFLECTION VOLTAGE INVENTORSZ MOOSHI R.NAMORDI, SVEND E. HAVN,

THEIR ATTORNEY.

M. R. NAMORDI ETAL 177 395 Apnl 1965 FLAT CATHODE RAY TUBE WITHINCREASED DEFLECTION SENSITIVITY Filed April 4, 1963 6 Sheets-Sheet 4 I$5; w 11 la ls l \V $5 s E $2 \l, Q n mm S souRcEoF VD 2 VERTICALDEFLECTION [VOLTAGE THEIR ATTORNEY.

April 6, 1965 Filed April 4, 1963 DEFLECTION VOLTAGE IN KILOVOLTS M. R.NAMORDI ETAL FLAT CATHODE RAY TUBE WITH INCREASED DEFLECTION SENSITIVITY6 Sheets-Sheet 5 FIGS 2 3 4 INCHES DISTANCE FROM TOP OF TARGET T0INTERSECTION WITH ELECTRON BEAM INVENTORS MOOSHI R. NAMORDI, SVEND E.HAVN,

THEIR ATTORNEY,

M. R. NAMORDI ETAL Apnl 1965 FLAT CATHODE RAY TUBE WITH INCREASED3177395 DEFLECTION SENSITIVITY 6 Sheets-Sheet 6 Filed April 4, 1963SOURCE OF VERTICAL DEFLECTION VOLTAGE INVENTORS MOOSHI R.NAMORDI, SVENDE. HAVN,

BY WJ- M THEIR ATTORNEY.

3,177,395 FLAT CATHODE RAY TUBE WITH INCREASED DEFLECTION SENSITIVITYMooshi R. Namordi, Syracuse, and Svend E. Havn,

Liverpool, N.Y., assignors to General Electric Company, a corporation ofNew York Filed Apr. 4,1963, Ser. No. 270,743

. 7 Claims. (Cl. 315-44) This invention relates to an image displaysystem and more particularly to electron beam deflection arrange-' mentsfor relatively shallow cathode ray tubes.

An image display system of the type to which this invention pertains isshown and described in cope'nding US. patent application Serial No.141,862, filed September 29, 1961, now Patent No. 3,155,872, which isassignedto the assignee of the present invention. In such a system, ashallow cathode ray tube is employed having a target area positioned ona front or viewing area of the tube and deflection means are spaced fromthe target area on the inside of an opposite wall of the picture tube.Suitable operating potentials are applied to the target area andto thedeflection means for providing a potential gradient therebetween suchthat when an electron beam is directed into the space which. separatesthe target area and the deflection means, the beam is deflected from oneextremity of the target to an opposite extremity of the target'in avertical direction.

United States Patent Horizontal deflection is accomplished by entranceof the electron beam into the space between the target area anddeflection means along one of a plurality of spaced parallel paths whichare substantially parallel to the target area. It is to be understoodthat the terms horizontal and vertical scanning are used in a relativesense, for convenience, to denote two mutually perpendicular directionsof scanning at the viewing screen.

In apparatus of the above character, the excursion of the vertical sweepvoltage which must be supplied for vertical scanning is of a highmagnitude, and normally is well in excess of a few thousand volts. Whilelow power static, or constant potential, voltages in the order of a fewthousand volts or more may be supplied by relatively inexpensive andwell-known circuits, a controlled voltage having excursions in thisorder of magnitude may be economically generated only by resorting totechniques bordering on the outermost fringes of the presentstateof-the-art. The problem becomes particularly acute when it issought to utilize semi-conductorcircuits, and it is highly desirablethat such circuits be utilized to more fully exploit the inherentadvantages to be gained by utilizing a compact, shallow cathode raytube. Therefore, it is desirable to minimize the required excursion ofthe vertishallow cathode ray tube having increased sensitivity toverticaldeflection voltage without requiring the generation ofadditional alternating current voltages in the system associatedtherewith.

A further object of this invention is to provide a shallow cathode raytube having increased sensitivity to vertical deflection voltage withoutincreasing the external dimensions of the tube.

Briefly, in accordance with one form of this invention, the entrance tothe throat of the cathode ray tube, or slit through which the collimatedelectron beam passes after horizontal deflection and prior to enteringthe space between the target areaand deflection means, is reduced insize to provide an increased voltage gradient for a given diiference inpotential between opposed surfaces defining the throat. The initialportion of the throat-defining surface which is disposed toward thefront or target side of the tube is then electrically insulated fromother conductive surfaces in the tube and maintained at a reduceddirect-current potential. The electron transit time through the initialportion of the throat region is thereby increased to provide a longertime for the field produced by the vertical deflection voltage toinfluence the electron beam. Since the influence of a given electricfield upon an electron passing therethrough is related directly to thetime during which the electron is in the field, the improved throat ofthis invention allows the field provided by the vertical deflectionvoltage to be lessened for a given desired deflection of the electronbeam. Thus, the sensitivity of the tube to vertical deflection voltageis increased.

The invention will be explained in more detail in connection with thedrawings in which:

FIGURE 1 is a front view of a shallow cathode ray tube;

FIGURE 2 is an external bottom view of FIGURE 1;

FIGURE 3 is a side view of FIGURE 1 at section AA, illustrating thepreferred embodiment of this invention; 7

FIGURE 4 is a side view similar to FIGURE 3, illustrating an alternativeembodiment of this invention;

FIGURE 5 presents -a graphical comparison of the eflfectiveness of thevarious embodiments of this invention; and

FIGURE 6 is a side view similar to FIGURE 3, illustrating anotheralternative embodiment of this invention.

By way of explanation, and not by way of limitation, a specific shallowcathode ray tube is now described to provide a background whichhighlights and more clearly shows the mode of operation and advantagesof the present invention. Referring to the specific form of a shallowcathode ray tube illustrated in FIGURES l and 2, the portion of the tubeopposite the bracket I is the image section; the portion opposite thebracket VD, the vertical deflection section; and the portion oppositethe bracket HD is the horizontal deflection and collimating section.

As seen in FIGURE 1, an electron gun 2 projects a beam of electrons 3between pole plates 4, only one of which is visible in the view. Themagnetic field set up between pole plates 4 is accomplished in awell-known manner by a yoke 6, deflection coil 8 and source 10 ofhorizontal deflection signal. The magnetic field causes the beam to scanin a plane parallel to the paper from an extreme position 3' to theother extreme position 3". Magnetic pole plates 12, only one of which isseen in FIGURE 1, are mounted adjacent the outside Wall of thehorizontal deflection section HD. Of course, various types of horizontaldeflection sections may be used; the one that is the subject matter ofthe US. patent application Serial No. 141,863 filed September 29, 1961,which is assigned to the assignee of the present invention ispreferable. In accordance with this preferred horizontal deflectionsection, pole plates 12 are joined at their righthand ends by a magnet14, and they are so shaped that the beam of electrons emerges alongparallel vertical paths all in the same plane regardless of where thebeam first enters the space between the pole pieces 12. The collimatedhorizontally scanned beam thus formed enters 69 the throat of thevertical deflection section VD and emerges therefrom to scan target 16.

Various types of image sections may be used; the one that is the subjectmatter of the US. patent application Serial No. 141,862, filed September29, 1961, now Patent No. 3,155,872, which is assigned to the assignee ofthe present invention is preferred. The image section of the shallowcathode ray tube illustrated in FEGURE 3 is in substantial conformitywith the teaching of the aforementioned disclosure. Componentspreviously described are indicated by the same numerals. The tubeenvelope is shown as being made of glass, though other suitablematerials may comprise various portions or all of the tube envelope. Inthe following discussion the term resistive coating or resistive meansrefers to a means having sufficient resistance so as to not unduly loadany source of voltage applied across it, and conductive coating, meansor surface refers to means which have an insignificant amount of voltagedifference between any two points thereof.

The target to may be comprised of the customary phosphor layer 18deposited on the inside of the front wall of the glass and an aluminumor other electron permeable metallic coating Zil along its top edge. Aresistive coating 24 is formed on the back wall and extends downwardlyuntil it makes an electrical connection at 27 with a conductive lining25 which extends down along the back wall into the vertical deflectionsection VD, terminating at the throat thereof. The conductive lining 26is a means for applying deflection voltage signals to the resistivecoating 24 and to the means for deflecting the beam in the lower regionof the target 16. A conductor 28 makes electrical contact with theresistive coatings 22 and 24 all along their intersection at the topright corner. In the particular embodiment of a shallow cathode ray tubeillustrated, one portion of the throat is defined by a conductive strip25 which may be an extension of conductive lining 26.

In accordance with the present invention the vertical deflectionsensitivity is increased by providing two electrically conductivecoatings 21 and 23 in the vertical deflection section of the tubeopposite lining 26 and strip 25. Coatings 21 and 23 are electricallyisolated and define together one wall of the vertical deflectionsection. Conductive coating 21 extends from adjacent target 16 towardthe entrance to the throat of the vertical deflection section.Preferably, coating 21 is an extension of conductive coating 2% which iscoextensive with the target 16. In such event, an appropriate voltagemay be supplied to both coatings 2d and 21 through lead 21 which extendsthrough the glass envelope and makes electrical connection with coating21. Of course, in the example given the lead 21' would be equallyeffective if connected to coating 29:, such as at the top cornerthereof.

In order to provide a suitable operating potential for conductivecoating 23, a lead such as 23' may extend through the envelope and makeelectrical connection with coating 23, as shown. Since the primaryobject of providing separate coatings 21 and 23 is to increase thetransit time of electron beam 3 by allowing coating 23 to have a lesserpotential than coating 21, it will be readily apparent that it is notessential that separate leads 21 and 23 be utilized. For example,coating 23 may be maintained at a lesser potential than coating 21 byproviding a suitable highly resistive internal connection betweencoating 21 and coating 23 and providing a suitable connection fromcoating 23 to an internal point of relatively lower potential or groundinside the tube.

In order to provide maximum vertical deflection sensitivity coating 21preferably extends from the lower portion of target 16 toward the backof the tube and downwardly in the form of a curve of progressivelyincreasing slope. Also, coating 23, while electrically isolated fromcoating 21, provides a geometrical extension (i.e. coating 23 is aspatial continuation of coating 21) of the surface .take a variety offorms.

formed by coating 21 and extends downwardly to termimate in a verticallydisposed strip 23 which defines with strip 25 the entrance to the throatof the cathode ray tube. The surface defined by coatings 21 and 23 mayIn a preferred embodiment the surface is generally characterized as aparabola.

By constructing the tube in accordance with the present invention,wherein coatings 21 and 23 are electrically isolated, it is possible toprovide two electric fields of relatively independent field strengths inthe vertical deflection section. This is achieved by providing voltagesof differing magnitudes to the respective coatings.

Generally, for reasons of economy and ease of manufacturing, coating 21is at the same voltage magnitude as coating 20 and preferably directlyconnected thereto. Therefore, the voltage supplied to coating 23 isvaried to provide the desired voltage difference. By selecting thisvoltage parameter to be less than the voltage of coating 21, thevelocity of the electrons in the throat of the vertical deflectionsection is lessened. Lessening the electron velocity in the throatcauses a longer transit time in this region, allowing a given electricfield therein to exert more influenceupon the path of the electron beam.Thus, by providing a longer electron transit time in the throat of thevertical deflection section the required variation in deflection isachieved by an electric field having lesser variations in strength,thereby requiring reduced voltage excursions to produce the field,resulting in a tube of increased sensitivity to vertical deflectionvoltage.

There are other parameters which may be varied in order to achieve aparticular desired result by utilizing the present invention. The width29 of the entrance to the throat defined by strips 23" and 25 may bevaried, and vertical deflection sensitivity is largely determined bythis parameter. Decreasing width 29 increases the vertical deflectionsensitivity. The relative areas in the vertical deflection sectioncovered by coatings 21 and 23 may be varied. In general, it has beenfound that increasing the dimension 30, which is the horizontal width,or horizontal projection of coating 23, increases the verticaldeflection sensitivity.

The primary criterion which limits selection of the aforementionedparameters in the direction of maximized vertical deflection sensitivityis associated with impingement of electron beam 3 upon coating 23,rather than upon target 16, when it is desired to scan the lower portionof target 16. The tendency of electron beam 3 to impinge upon coating 23also increases as the aforementioned parameters are varied to achievemaximum vertical deflection sensitivity. It is apparent that a greatnumber of varied combinations and permutations of these parameters maybe utilized successfully to achieve improved vertical deflectionsensitivity.

Particularly effective results have been obtained with the shallowcathode ray tube illustrated in FIGURE 3 by providing a width 29 equalto inch, by making dimension 30 substantialy equal to width 29 and byproviding a potential of seven kilovolts to coating 23 through lead 23'.In such a shallow cathode ray tube, satisfactory vertical scanning wasachieved by applying to conductive lining 26 through lead 26', avertical deflection voltage from source 32 having maximum limits ofexcursion of 6.1 and 1.8 kilovolts. Cathode ray beam 3 was therebycaused to vary between path 33 and path 31 to effect a complete verticalscan as the voltage from source 32 varied from 6.1 kilovolts to 1.8kilovolts.

By way of pointing out the improved vertical deflection sensitivityachieved by using the arrangement of the present invention, a shallowcathode ray tube of structure similar to that of FIGURE 3, but having asingle equipotential surface extending from coating 20, rather than theseparate coatings 21 and 23 of this invention, required a verticaldeflection voltage varying between the limits of 8.2 kilovolts and 1.6kilovolts. The controlled voltage excursion required by the shallowcathode ray tube not constructed in accordance with the presentinvention was thus 6.6 kilovolts, whereas the controlled voltageexcursion which provides similar results using the teaching of thisinvention is only 4.3 kilovolts, a reduction of over 35%. Such asubstantial increase in vertical deflection sensitivity greatly lessensthe burden cast upon the generator circuit of source 32.

FIGURE 4 illustrates an alternative embodiment of this invention. Forthe purpose of showing that the invention is not dependent upon aspecific variety of shallow cathode ray tube, the tube shown in FIGURE 4has been selected different from the tube of FIGURE 3. In particular,the tube of FIGURE 4 features a resistive coating 41 formed on the backwall and extending downwardly to the entrance of the throat of thevertical deflection section where coating 41 is connected to aconductive strip 42. Source 32 of vertical deflection voltage isconnected by conductor 42 to conductive strip 42. It will be noted thatthis structure differs from that of FIGURE 3 wherein resistive coating24 extended downwardly to cover only a portion of the back wall and wasconnected at 27 to a conductive lining 26 which extended down theremaining portion of the back wall.

In the embodiment of this invention shown in FIGURE 4, a conductivecoating 43 is connected electrically to conductive coating 20 near thelower region of target 16. Preferably, conductive coating 43 is anextension of conductive coating 20. Coating 43 extends from the lowerregion of target 16 backwardly and downwardly in a curve ofprogressively increasing slope to a termination adjacent the entrance ofthe throat of the vertical deflection section. Horizontally spaced fromthe termination of conductive coating 43 is a conductive strip 44, whichtogether with strip 42 defines the entrance to the throat of thevertical deflection section. The throat width is defined by dimension45, which is the horizontal distance between strips 44 and 42.

In order to point out the advantages of the two specifically illustratedembodiment of this invention, as well as to provide a comparison of therelative effectiveness of the two specific embodiments, reference is nowhad to FIGURE 5. In the interest of providing a controlled comparison,the curves of FIGURE 5 are plotted from data obtained using a shallowcathode ray tube of the type shown in FIGURE 4. In the graph of FIGURE5, deflection voltage in kilovolts is plotted on the ordinate and theposition at which the electron beam impinges upon or intersects with thetarget is measured on the abscissa as distance from the top of thetarget.

The distance from the top to the bottom of target 16 is four inches, andit will be noted that the abscissa of the graph of FIGURE 5 showsdeflection voltages for a complete vertical scan of the target. Thedifference between the deflection voltage at extremes of each curve is ameasure of the vertical deflection sensitivity of, the particularembodiment represented by the curve. Curve 51 shows the deflectionvoltage sensitivity of a shallow cathode ray tube of the type shown inFIGURE 4, but not incorporating the present invention. Specifically, the

. curves constructed from data obtained by utilizing a continuousconductive strip extending from the bottom of target 16 and describing agenerally parabolic curved path downwardly to the entrance of the throatof the vertical deflection section. No conductive strip, such as 44, wasused and the width of the entrance to the throat was equal to one-halfinch. A vertical deflection voltage having a peak-to-peak excursion of9.3 kilovolts was required to obtain a complete vertical scan of target16.

Curve 52 shows the vertical deflection sensitivity of a shallow cathoderay tube constructed as shown in FIG- URE 4, and having a dimension 45,of the entrance to the throat of the vertical deflection section, equalto fourtenths of an inch. However, for curve 52 the potential of strip44 was made equal to that of conductive coatings 20 and 43, that is tosay, ten kilovolts. Curve 52, then, serves primarily to show thesubstantial increase in vertical deflection sensitivity achieved bynarrowing the entrance to the throat. It can be seen that the .verticaldeflection voltage excursion, peak-to-peak, is 7.1 kilovolts. By thischange alone, the vertical deflection sensitivity has been increased bymore than 20% over that of the prior embodiment characterized by curve51.

Curve 53 is constructed from data taken with a shallow cathode ray tubeconstructed in accordance with and supplied with the potentialsindicated in FIGURE 4. The potential of conductive strip 44 was loweredfrom ten kilovolts as used in the tube characterized by curve 52, toseven kilovolts. The result may be seen to be a further decrease in therequired peak-to-peak excursion of the vertical deflection voltage. Asshown, the value is 6.3 kilovolts. The vertical sensitivity is increasedmore than 30% curve 51.

Curve 54 is plotted from data obtained by utilizing the embodiment ofthis invention shown in FIGURE 6, wherein a shallow cathode ray tube isused of the type wherein the resistive coating extends completely downthe back wall of the image section, as in FIGURE 4. It is apparent thatthe vertical deflection section of the tube otherwise is similar to thatshown in FIGURE 3. The embodiment of FIGURE 6 is more effective inincreasing vertical deflection sensitivity than the specific embodimentof this invention shown in FIGURE 4. In particular, the peak-to-peakexcursion of the deflection voltage has been reduced to 5.7 kilovolts,almost a 40% decrease in the required deflection voltage excursion 0fthe arrangement characterized by curve 51.

' Finally, the dashed curve 55 is shown to provide more completedisclosure of the vertical deflection voltage characteristics of theshallow cathode ray tube structure of FIGURE 3, utilizing the embodimentof this invention shown therein, as discussed before. The improvement invertical deflection volt age sensitivity indicated by curve 55, whencompared to curve 54, results from the more effective tube structure ofFIGURE 3, as contrasted to the tube structure of FIGURE 6, and forms nopart of the present invention. Specifically, the use of a conductiveportion on the back wall of the image section is part of the subjectmatter of the aforementioned US. patent application Serial No. 141, 862,now Patent No. 3,155,872.

There has been shown and described herein an improved verticaldeflection system for a shallow cathode ray tube. The invention has beendescribed in conjunction with two specific embodiments of a shallowcathode ray tube. It will be apparent to those skilled in the .art thatthe teaching of this invention may be applied to other analogouselectron-optic systems to obtain the improved results shown herein.Also, while this invention has been described with reference to twospecific embodiments thereof, it should be understood that manymodifications and variations of the specific structure disclosed willoccur to those skilled in the art, and it is therefore intended that thescopeof this invention be defined solely by reference to the followingclaims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is as follows:

1. In a shallow cathode ray tube system having an electron target,deflection means disposed in spaced relationship opposite said targetand defining a space therebetween, a source of electrons for generatingan electron beam generally parallel to said target and a verticaldeflection section including a throat through which the beam passesprior to entry into the space between said target and deflection meansand impinging on said target, the improvement comprising:

(a) first conductive means extending from adjacent said target towardthe entrance to the throat of said section and defining one portion of awall of said section;

above the arrangement characterized by (b) second conductive means,electrically isolated from said first conductive means, and defining asecond portion of said wall including the entrance to said throat; and

(0) means arranged to maintain said second conductive means at a lowervoltage level than said first conductive means, whereby the transit timeof electrons in said throat is increased and the vertical deflectionsensitivity of said tube is improved.

2. In a shallow cathode ray tube system having an electron target,deflection means disposed in spaced relationship opposite said targetand defining a space, therebetween, a source of electrons for generatingan electron beam generally parallel to said target and a verticaldeflection section including a throat through which the beam passesprior to entry into the space between said target and deflection meansand impinging on said target, the improvement comprising:

(a) first conductive means extending from adjacent said target towardthe entrance to the throat of said section and defining the majorportion of a wall of said section;

(11) second conductive means, electrically isolated from said firstconductive means, and disposed in the entrance to the throat of saidsection; and

(c) means arranged to maintain said second conductive means at a lowervoltage level than said first conductive means, whereby the transit timeof electrons in said throat is increased and the vertical deflectionsensitivity of said tube is improved.

3. In a cathode ray tube system having an image section comprising anelectron target and a conductive electron-permeable coating on saidtarget, deflection means disposed in spaced relationship opposite saidcoating and defining a space therebetween, a source of electrons forgenerating an electron beam generally parallel to said target, and avertical deflection section including a throat through which theelectron beam passes prior to entry into the space between said targetand coating and impinging on. said target, the improvement comprising:

(a) first conductive means connected to said coating and extendingtoward the entrance to the throat of said vertical deflection section todefine a portion of one wall of said section;

(b) second conductive means electrically isolated from said firstconductive means, and extending into the entrance to said throat todefine another portion of said wall; and

(0) means arranged to maintain said second conductive means at a lowervoltage level than said first conductive means, whereby the transit timeof electrons in said throat is increased and the vertical deflectionsensitivity of said tube is improved.

4. The cathode ray tube of claim 3 wherein said first conductive meansdefines a first surfiace of progressively increasing slope extendingfrom said target toward said throat.

5. The cathode ray tube of claim 4 wherein said second conductive meansdefines a second surface which is a geometrical extension of said firstsuriiace and which extends to the entrance portion of said throat.

6. The cathode ray tube of claim 5 wherein the horizontal width of saidsecond conductive means is substantially equal to the width of theentrance to said throat.

7. The cathode ray tube of claim 5 wherein said first and secondconductive means define a surface having a generally paraboliccross-section.

References Cited by the Examiner UNITED STATES PATENTS DAVID G.REDINBAUGH, Primary Examiner.

1. IN A SHALLOW CATHODE RAY TUBE SYSTEM HAVING AN ELECTRODE TARGER,DEFLECTION MEANS DISPOSED IN SPACED RELATIONSHIP OPPOSITE SAID TARGETAND DEFINING A SPACE THEREBETWEEN, A SOURCE OF ELECTRONS FOR GENERATINGAN ELECTRON BEAM GENERALLY PARALLEL TO SAID TARGET AND A VERTICALDEFLECTION SECTION INCLUDING A THROAT THROUGH WHICH THE BEAM PASSESPRIOR TO ENTRY INTO THE SPACE BETWEEN SAID TARGET AND DEFLECTION MEANSAND IMPINGING ON SAID TARGER, THE IMPROVEMENT COMPRISING: (A) FIRSTCONDUCTIVE MEANS EXTENDING FROM ADJACENT SAID TARGET TOWARD THE ENTRANCETO THE THROAT OF SAID SECTION AND DEFINING ONE PORTION OF A WALL OF SAIDSECTION; (B) SECOND CONDUCTIVE MEANS, ELECTRICALLY ISOLATED FROM SAIDFIRST CONDUCTIVE MEANS, AND DEFINING A SECOND PORTION OF SAID WALLINCLUDING THE ENTRANCE TO SAID THROAT; AND (C) MEANS ARRANGED TOMAINTAIN SAID SECOND CONDUCTIVE MEANS AT A LOWER VOLTAGE LEVEL THAN SAIDFIRST CONDUCTIVE MEANS, WHEREBY THE TRANSIT TIME OF ELECTRONS IN SAIDTHROAT IS INCREASED AND THE VERTICAL DEFLECTION SENSITIVITY OF SAID TUBEIS IMPROVED.